1. Field of the Invention
This invention relates to curable epoxy resins having increased thermal shock resistance; and, more particularly, to certain anhydride cured epoxy resins containing certain polyether diamide additives.
2. Prior Art
Epoxy resins constitute a broad class of polymeric materials having a wide range of physical characteristics. The resins are characterized by epoxide groups which are cured by reaction with certain catalysts or curing agents to provide cured epoxy resin compositions with certain desirable properties. One such class of curing agents is generally the anhydrides. The most commonly used anhydride curing agents are difunctional materials such as maleic anhydride, phthalic anhydride and the like, as well as tetrafunctional materials such as pyromellitic dianhydride.
It is known to use polyamides as epoxy curing agents. Simple amides such as acetamide, benzamide and adipamide have been used, but low activity and/or solubility require use of basic catalysts. The advantages and diadvantages of polyamides as curing agents is discussed in Lee, Henry and Neville, K. Handbook of Epoxy Resins, McGraw Hill Book Co., New York, 1967. but, generally, the hydrogen of the primary or secondary amides is weakly reactive with epoxy groups.
Epoxy resins for casting, embedding or encapsulating etc. must withstand repeated cycles of high and low temperatures without cracking. However, lowering the temperature increases stress due to shrinkage and reduces the ability of the resin to flow, thus relieving the stress.
Anhydride cured resins are useful in applications where high heat deflection is required. However, such materials exhibit brittleness and thus a low resistance to thermal shock. Diluents and modifiers do improve thermal shock resistance properties but, unfortunately, adversely influence the heat deflection properties as shown in May and Tanaka, Epoxy Resins, New York, 1973, p. 299. Likewise, plasticizers have not found wide acceptance in epoxy technology primarily because most of them are incompatible with the cured resins.
It has now been unexpectedly found that a specific diamide terminated polyoxyalkylene material having a molecular weight of from about 2000 to about 3000, when employed as an epoxy additive, provides cured epoxy resin compositions exhibiting outstanding thermal shock resistance. Specifically, epoxy resins incorporating these additives, upon curing with a specific alkyl substituted bicyclic vicinal anhydride curing agent, provide a material with high heat deflection and superior resistance to thermal shock.